General keyboard units used as input equipment in computer systems or word processors are provided with indication functions by light emission, to inform an operator of various states of the keyboard, or of the systems wherein the keyboard units are used, for example, "SYSTEM READY", "POWER ON", "HOLD", "FAULT" and the like, in addition to keys for letters and numerals and the like, and key switches corresponding to the keys. .p FIG. 1 is a simplified diagram of a conventional keyboard unit. The keyboard unit shown in FIG. 1 consists of a keyboard comprising a plurality of keys switches 11 which are placed in correspondence to keys and form a matrix, and light-emitting devices (LED) 21-1, 21-2; a printed circuit board (PCB) comprising a controller 3, a decoder 10, drivers 12-1, 12-2, 12-3, 22-1, 22-2, receivers 13-1, 13-2, resistors 4-1, 4-2, 4-3, 4-4, and an output register 20; and wiring portions comprising wires connecting the keyboard and the PCB. From the view point of the functions of the keyboard unit, the keyboard unit is divided into a key switch read-out portion 1' for reading the key switches 11, a light-emitting display portion 2' having a display function of informing an operator of the aforementioned various states by light emission, and a controller 3 controlling the key switch read-out portion 1' and the light-emitting display portion 2'. The constructions and the functions of the above three portions are described hereinafter.
The controller 3, which has a microprocessor, outputs a key address signal to the key switch read-out portion 1' for reading out which key is pushed, and receives the results of the reading. The controller 3 also controls the light emission by the LEDs 21-2, 21-2 in the light-emitting display portion 2.
The key switch read-out portion 1' comprises: a plurality of key switches 11 which are placed in correspondence to the keys and form a matrix; the diodes accompanying the key switches; drivers 12-1, 12-2, 12-3 provided in correspondence to the lines of the matrix; the receivers 13-1, 13-2 provided in correspondence to the rows of the matrix; the resistors 4-1, 4-2, each of which is connected at one terminal to the input side of the corresponding one of the receivers 13-1, 13-2, and at the other terminal to a voltage source of a higher voltage (+5V); and a decoder 10. One end of each key switch 11 in each line of the matrix is connected through a diode to the output of the corresponding one of the drivers 12-1, 12-2, 12-3, and the other end of each key switch 11 in each row of the matrix is connected to the input of the corresponding one of the receiver 13-1, 13-2.
The aforementioned key address signal from the controller 3 selects one of the drivers 12-1, 12-2, 12-3, each of which corresponds to one of the lines in the matrix. The meaning of the key address signal is decoded at the decoder 10, and the decoder 10 outputs a drive input to the selected driver among the drivers 12-1, 12-2, 12-3. Upon receiving the drive input, the output of the selected driver among the drivers 12-1, 12-2, 12-3 is brought to a lower voltage level.
At this time, if none of the key switches 11 connected through the diodes to the outputs of the drivers 12-1, 12-2, 12-3 is closed, all input of the receivers 13-1, 13-2 remain at a higher voltage level (+5V), but if any of the key switches 11 connected to the outputs of the drivers 12-1, 12-2, 12-3 is closed, the input of the receiver 13-1 or 13-2 to which the aforementioned other end of the key switch is connected, is brought to the lower voltage level, and thus the position of the operated key on the keyboard is read.
The diodes connected in series to the key switches 11 are provided to prevent a misreading due to the forming of a plurality of the circuits through a plurality of the key switches when a plurality of the key switches are operated at the same time.
The aforementioned light-emitting display portion 2' comprises the output register 20, the drivers 22-1, 22-2 for the light emitting devices (LED) 21-1, 21-2, the light-emitting devices (LED) 21-1, 21-2, and the resistors 4-3, 4-4. A command for an on-and-off control of each of the LEDs 21-1, 21-2 from the controller 3 is sent through the output register 20 to the drivers 22-1, 22-2. Upon receiving the command input from the controller 3, the output of each of the drivers 22-1, 22-2 is brought to the lower level. Each output of the drivers 22-1, 22-2 is connected through the corresponding one of the resistors 4-3, 4-4 to one end of the corresponding one of the LEDs 21-1, 21-2, and the other ends thereof are connected to a voltage source of a higher voltage +5V.Therefore, when the output of any of the drivers 22-1, 22-2 is brought to the lower voltage, the corresponding LED 21-1, 21-2 is turned on and emits light. The outputs of the drivers 22-1, 22-2 which do not receive the command from the controller 3 remain at the higher level, and therefore, the corresponding LEDs 21-1, 21-2, which are connected through the corresponding resistors 4-3, 4-4 to the outputs of the drivers 22-1, 22-2, are not turned on, and do not emit light.
In the above mentioned keyboard equipment, the resistors 4-1, 4-2 are provided at the input side of the receivers 13-1, 13-2 in the key switch read-out portion 1', and the inputs of the receivers 13-1, 13-2 are connected through the resistors 4-1, 4-2 to the +5V voltage source. On the other hand, in the light-emitting display portion 2', one end of each of the LEDs 21-1, 21-2 is connected to the corresponding one of the resistors 4-3, 4-4, and the other end is connected to the +5V voltage source.
In this situation, if the resistors connected to the +5V voltage source and the wires accompanying the resistors are commonly used for both the key switch read-out portion 1' and the light-emitting display portion 2', the number of necessary resistor and wires can be reduced.
Nevertheless, if the above one end of each of the LEDs 21-1, 21-2 is connected to the input level of the corresponding one of the receivers 13-1, 13-2, when one of the LEDs 21-1, 21-2 is turned on, the input level of the corresponding one of the receivers 13-1, 13-2 becomes lower than the +5V level due to a voltage drop through the corresponding one of the resistors 4-1, 4-2, and this may prevent a correct detection by the receivers 13-1, 13-2 of the OFF state of the key switch 11.